Answer:
The length of ab is 10 inch 
Explanation:
The line 'ab' is shown in figure -1 
Given : 
bf=3 in and fa=7 in
we need to find out the length of 'ab'
so, 
ba =bf+fa
Since, bf=3 in and fa=7 in
ba =3 in+7 in
ba =10 in
Therefore, the length of 'ba' is 10 inch